<!DOCTYPE HTML>
<html>

<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>计时器</title>
  <script type="text/javascript">
    var num = 0;

    function startCount() {
      document.getElementById('count').value = num;
      num = num + 1;
    }
    setTimeout(startCount(), 3000)
  </script>
</head>

<body>
  <form>
    <input type="text" id="count" onclick="startCount()" />
  </form>
</body>

</html>